1. Field of the Invention
The present invention relates to a charging member for developing an electrostatic latent image which is used to develop an electrostatic latent image formed by an electrostatic recording method in electrophotography, a method for manufacturing the charging member and an electrostatic latent image developer, image forming apparatus and image forming method using the charging member.
2. Description of the Related Art
When conventionally developing an electrostatic latent image formed by an electrostatic recording method, a charging member is used to impart an appropriate amount of positive or negative charge to a toner. Particularly in the case of using a two-component developer, a coated carrier coated with a resin which is the charging member is used. However, the use of the two-component developer causes problems such as a reduction in charge amount at high temperature and humidity, an extreme increase in charge amount at low temperature and humidity, and also gradual deterioration of charge controllability due to contamination of the surface of a carrier by toner components, e.g. a binder resin, charge controlling agent, and external additives. In order to solve such problems, a coated carrier is used in which the carrier is produced by coating a core material (hereinafter called xe2x80x9ccarrier corexe2x80x9d or simply called xe2x80x9ccorexe2x80x9d as the case may be) with such as a fluorine-based polymer, silicone-based polymer, silicone oil, which has a low surface energy, and is superior in environmental stability and surface contamination resistance to conventional materials. In recent years, carriers have been proposed which are coated with a fluorine-based polymer as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 49-51950: 57-99653 and 60-202451; with a silicone-based polymer as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 60-19156: 62-121463: 61-110159 and 61-110160; and with a resin containing silicone oil or the like as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 3-46669: 3-46670: 3-46671 and 5-72814.
These carriers, however, require further improvements. They have insufficient adhesiveness to the carrier core, insufficient wear resistance of a resin coating layer, and show poor environmental stability due to large variations in the initial charge values and the spread of the charge distribution of a toner at high temperature and humidity or at a low temperature and humidity, particularly, due to reduction in charge amount at high temperature and humidity, or an extreme increase in the amount of charge at a low temperature and humidity. Carriers coated with an amide-type polymer are also proposed as described in Japanese Patent Application Laid-Open (JP-A) Nos. 3-217857 and 3-219263, but these carriers still require further improvements in environmental stability and charging properties since amino groups in the resin are used for the crosslinking reaction.
Further, a carrier produced by coating a core material with a phenol resin is used to improve the durability of the carrier. This carrier, however, has insufficient charging properties because a phenol resin with no substituent group is generally used.
On the other hand, as a developing sleeve, a metal sleeve such as aluminum and stainless steel (SUS) or an elastic sleeve such as silicone rubber, NBR and EPDM have been used.
However, control of initial charging of a toner is difficult due to the low charging ability of these developing sleeves themselves, and further, an amount of low charged toner or reversely polarized toner increase with the increase in the number of copies sheets due to the deterioration of the toner and contamination of the developing sleeve arising from long term use, thereby resulting in the easy occurrence of problems such as increase in fog or reduction of image density.
In order to solve these problems, many attempts have been made to increase the toner charge in the initial stage. For instance, the surface of the developing sleeve has been coated with a charge control material exhibiting charge with the polarity opposite to that of the toner. In order to assist the charging properties of a toner with a negative charge, trial methods in which the developing sleeve is coated with a resin exhibiting positive charging properties such as an acrylic resin and nylon resin and the like, or in which the developing sleeve is coated with the above mentioned resins or other resins having non-charging-properties, e.g. a phenol resin, containing a positive charge control agent such as a quaternary ammonium salt and the like, have been tried. Particularly the phenol resin has high mechanical strength, wear resistance and durability and is hence a desirable material to coat the developing sleeve therewith.
However, though these developing sleeves certainly assist in promoting the charging speed of the toner and in increasing the charge amount of the toner in the initial stage, the developing sleeves possess poor environmental stability, and problems such as reductions in image density and generation of ghost images due to excessive charge in the vicinity of the surface thereof and duplication of charged layers arise, especially at low temperature and humidity.
Further, when coating the developing sleeve with a phenol resin containing a positive charge control agent, neither durability nor environmental stability can be sufficiently imparted when only phenol resin is used, even if environmental stability is high.
An attempt to coat the surface of the developing sleeve with a silicone resin containing an aminosilane coupling agent is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 1-147478. The attempt to coat the surface of the developing sleeve with a resin having a functional group containing a nitrogen atom such as an amino group is surely effective for increasing charge. However, reduction in charge amount may occur for extremely long term use even in thus method and thus such problems still has to be solve for obtaining a stable image quality.
The present invention has been achieved in view of the above drawbacks in prior art. An object of the present invention is to provide a highly durable charging member which ensures rapid charging, prevents a reduction in the charge amount at a high temperature and humidity, and prevents a extreme increase in the charge amount at a low temperature and humidity, provides a carrier capable of preventing a developer from deteriorating due to peeling of the coating from the core, and from deteriorating due to spent carrier caused by aging of the toner. The object also includes providing a method for manufacturing the charging member as well as providing an electrostatic latent image developer, an image forming apparatus and an image forming method each using the charging member. The present inventors have conducted extensive studies to solve the above problems, and found that these problems can be solved by using a phenol resin possessing the specific structure, shown below.
According to a first aspect of the present invention, there is provided a charging member comprising a resol-type phenol resin having at least one structural unit selected from the group consisting of structural units represented by the structural formulas (I) to (VIII): 
wherein X1 to X6 in the structural formula (I), X1 to X8 in the structural formula (II) or X1 to X10 in the structural formulas (III) to (VIII) represent at least two bonding groups, one or more OH groups, one to four xe2x80x94[(CH2)pNR1R2] groups, the remainder represent a hydrogen atom, a halogen atom or an alkyl or an alkoxy group having 1 to 6 carbon atoms, wherein R1 and R2 each independently represents an alkyl group having 1 to 10 carbon atoms and p denotes an integer from 0 to 10.
According to another aspect of the present invention, there is provided a developing sleeve comprising a coating layer applied to the outer peripheral surface of a cylinder support, the coating layer composed of a charging member, on the outer peripheral surface of a cylindrical support, containing a resol-type phenol resin having at least one structural unit selected from the group consisting of structural units represented by the above structural formulas (I) to (VIII). According to a further aspect of the present invention, there is provided an electrostatic latent image developer comprising a toner and a carrier wherein the carrier is an electrostatic latent image developing carrier produced by coating the surface of a core material with a charging member including a resol-type phenol resin having at least one structural unit selected from the group consisting of structural units represented by the above structural formulas (I) to (VIII).
According to a still further aspect of the present invention, there is provided an image forming method comprising a step of forming an electrostatic latent image on an electrostatic latent image carrying member, a step of forming a developer layer on a developing sleeve and a step of developing the electrostatic latent image on the electrostatic latent image carrying member using the developer layer; wherein the developer consists of a toner and a carrier, wherein the carrier is an electrostatic latent image developing carrier produced by coating the surface of a core material with a charging member including a resol-type phenol resin having at least one structural unit selected from the group consisting of structural units represented by the above structural formulas (I) to (VIII).
According to a still further aspect of the present invention, there is an image forming method comprising a step of forming an electrostatic latent image on an electrostatic latent image carrying member, a step of forming a developer layer on a developing sleeve and a step of developing the electrostatic latent image on the electrostatic latent image carrying member using the developer layer, wherein the developing sleeve comprises a coating layer, on the outer peripheral surface of a cylinder support, composed of a charging member containing a resol-type phenol resin having at least one structural unit selected from the group consisting of structural units represented by the above structural formulas (I) to (VIII).
According to a still further aspect of the present invention, there is provided an image forming apparatus comprising a means of forming an electrostatic latent image on an electrostatic latent image carrying member, a means of forming a developer layer on a developing sleeve and a means of developing the electrostatic latent image on the electrostatic latent image carrying member using the developer layer, wherein the developing sleeve comprises a coating layer, on the outer peripheral surface of a cylindrical support, composed of a charging member containing a resol-type phenol resin having at least one structural unit selected from the group consisting of structural units represented by the above structural formulas (I) to (VIII). According to yet another aspect of the present invention, there is provided a method for manufacturing a charging member comprising a resol-type phenol resin having at least one structural unit selected from the group consisting of structural units represented by the above structural formulas (I) to (VIII) described below, wherein the charging member is produced by reacting at least one phenol derivatives selected from the group consisting of phenol derivatives represented by the structural formulas (IX) to (XVI) with formaldehyde or a material having the same effect as that of formaldehyde in an amount of 2 to 20 molar equivalents with respect to the phenol derivative and at a pH ranging from 8 to 12 in an aqueous ammonia or alkali hydroxide solution in an amount equivalent by mol to the number of hydroxyl groups of at least one phenol derivative: 
wherein X1 to X6 in the structural formula (IX), X1 to X8 in the structural formula (X) or X1 to X10 in the structural formulas (XI) to (XVI) represent one or more OH group, one to four xe2x80x94[(CH2)pNR1R2] groups and the remainder represents a hydrogen atom, a halogen atom, or an alkyl or alkoxy group having 1 to 6 carbon atoms, R1 and R2 each independently represent an alkyl group having 1 to 10 carbon atoms and p denotes an integer from 0 to 10.
In the present invention, the charging member is a material imparting charge to a carrier for developing an electrostatic latent: image in a two-component developer, to a developing sleeve in either a two- or one-component system in a developing apparatus, and to a toner.
The present invention will now be explained in detail.
In the present invention, the resol-type phenol resin having at least one structural unit selected from the group consisting of structural units represented by the following formulas (I) to (VIII) possesses the structural features in which, particularly, the group xe2x80x94[(CH2)pNR1R2] directly connected to an aromatic ring is contained in the structural unit. Therefore, the resin-type phenol resin has high charging imparting capability and excellent environmental stability whereby it can impart stable charge to the toner, thus enables to maintain an image of high quality even when used for a long period of time in electrophotography. 
wherein X1 to X6 in the structural formula (I) X1 to X8 in the structural formula (II) or X1 to X10 in the structural formulas (III) to (VIII) represent at least two bonding groups, one or more OH groups, one to four xe2x80x94[(CH2)pNR1R2] groups, the remainder represent a hydrogen atom, a halogen atom or an alkyl or an alkoxy group having 1 to 6 carbon atoms, wherein R1 and R2 each independently represents an alkyl group having 1 to 10 carbon atoms and p denotes an integer from 0 to 10.
The xe2x80x9cbonding groupxe2x80x9d in the present invention is a moiety produced by the reaction of the phenol derivative described below with, for example, formaldehyde in the presence of an aqueous ammonia or alkali hydroxide. The moiety has primarily a formula of xe2x80x94CH2OH, which reacts to impart thermosetting property.
These resol-type phenol resins in the present invention are compounds produced using at least one phenol derivative selected from the group consisting of phenol derivatives represented by the following structural formulas (IX) to (XVI): 
wherein X1 to X6 in the structural formula (I), X1 to in the structural formula (II) or X1 to X10 in the structural formulas (III) to (VIII) represent at least two bonding groups, one or more OH groups, one to four xe2x80x94[(CH2)pNR1R2] groups, the remainder represent a hydrogen atom, halogen atom or an alkyl or an alkoxy group having 1 to 6 carbon atoms, wherein R1 and R2 each independently represents an alkyl group having 1 to 10 carbon atoms and p denotes an integer from 0 to 10.
Concrete examples of the phenol derivatives in the present invention are given below in Tables 1 to 10, but the compounds in the present invention are not limited by these examples:
In order to produce the resol-type phenol resin using these derivatives, it is necessary that the aromatic rings in the above structural formulas (IX) to (XVI) have 2 to 8 bonding groups to make hardening possible. That is, the structural unit of the resol-type phenol resin obtained by using, as the raw material, the phenol derivatives represented by the above structural formulas (IX) to (XVI) may be expressed by the following general formula (XVII):
Z(CH2OH)yGeneral formulaxe2x80x83xe2x80x83(XVII)
The above formula represents at least one structural unit selected from the group consisting of the structural units represented by the structural formulas (I) to (VIII) and y denotes an integer from 2 to 8.
In the present invention, a method of manufacturing the resol-type phenol resin using the above phenol derivative as the raw material is preferably the method described below.
At least one phenol derivative selected from the group consisting of phenol derivatives represented by the structural formulas (IX) to (XVI) is allowed to react with formaldehyde or another material having the same effect as that of formaldehyde in an excessive amount to that of the phenol derivative, preferably 2 to 20 molar equivalents, more preferably 4 to 10 molar equivalents in an aqueous ammonia or alkali hydroxide solution in an amount equivalent by mol to the number of hydroxyl groups of the phenol derivative.
This reaction is carried out in the aqueous medium while the pH is maintained in a range of from 8 to 12.
When the amount of formaldehyde is less than 2 molar equivalents, introduction of the bonding group will be insufficient and this may result in poor hardening in the heat curing step. On the contrary, when the amount of formaldehyde exceeds 20 molar equivalents, crosslinking proceeds during the reaction, thereby resulting in forming a gel-like material.
When the pH is less than 8, the addition reaction of the phenol derivatives with formaldehyde becomes difficult, whereas when the pH exceeds 12, phenolate anions are oxidized and coloration of products takes place.
In this case, the reaction temperature is preferably in the range of 60xc2x0 C. to 80xc2x0 C. When the temperature is less than 60xc2x0 C., the resin producing reaction gets slow. When the temperature exceeds 80xc2x0 C., crosslinking reaction proceeds rapidly and cause gelation.
Given as examples of the compound having the same effect as that of formaldehyde are paraformaldehyde, hexamethylenetetramine, formalin gas, and an aqueous formalin solution. Among these, paraformaldehyde, hexamethylenetetramine, and an aqueous formalin solution are particularly preferable in view of reactivity.
As examples of the alkali hydroxide used in the present invention in an equimolar amount with the number of hydroxyl groups of the above phenol derivative, sodium hydroxide, potassium hydroxide, and sodium carbonate are given.
After completion of the reaction, the required resol-type phenol resin is obtained by neutralization of the reaction solution. As examples of the aqueous medium used in the reaction, water, alcohols having 1 to 4 carbon atoms such as methyl alcohol and ethyl alcohol, and mixtures of these are preferable.
The electrostatic latent image developing carrier acting as the charging member of the present invention can be obtained when an appropriate core material is coated with the above resol-type phenol resin.
The above resol-type phenol resin as such a coating resin for the electrostatic latent image developing carrier is coated on the core material like powders of magnetic metals such as iron, copper, nickel and cobalt which have a volume average particle size of 10 to 150 xcexcm or materials produced by dispersing these powders in a resin, and magnetic oxide powders such as magnetite and ferrite, or materials produced by dispersing these powders in a resin. The amount of the coating is generally 0.1 to 10.0% by weight, preferably 0.5 to 8.0% by weight with respect to the amount of the core. A resistance control agent such as carbon black, stannic oxide, or titanium oxide may be added to the coating layer to control resistance.
As a method for coating the core surface with the resol-type phenol resin prepared from the above phenol derivative as the raw material, known techniques such as a dipping method in which a core powder is dipped in a coating layer forming solution, a spraying method in which a coating layer forming solution is sprayed on the surface of a core, a fluidized bed method in which a coating layer forming solution is sprayed on a core material while the core material is floated in fluid air, or a kneader-coater method may be used. Further, it is necessary that the resol-type phenol resin component is hardened and crosslinked by heating after it is coated. The heating temperature at this time is preferably 100xc2x0 C. to 200xc2x0 C., more preferably 120xc2x0 C. to 180xc2x0 C. to harden the above resol-type phenol resin sufficiently.
The carrier obtained in this manner is mixed with a toner and used as a two-component developer. The toner can be obtained in the usual manner by melting and kneading a binder resin, a colorant and other additives, then cooling and pulverizing and finally by classifying if necessary.
Examples of the binding resin for the toner include monopolymers or copolymers of styrenes such as styrene and chlorostyrene; mono-olefins such as ethylene, propylene, butylene and isoprene; vinyl esters such as vinyl acetate, vinyl propionate, vinyl benzoate and vinyl acetate; xcex1-methylene aliphatic monocarboxylates such as methyl acrylate, ethyl acrylate, butyl acrylate, dodecyl acrylate, octyl acrylate, phenyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate and dodecyl methacrylate; vinyl ethers such as vinyl methyl ether, vinyl ethyl ether and vinyl butyl ether; and vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone and vinyl isopropenyl ketone.
Typical examples of the binder resin are polystyrene, styrene-acrylate copolymer, styrene-methacrylate copolymer, styrene-acrylonitrile copolymer, styrene-butadiene copolymer, styrene-maleic anhydride copolymer, polyethylene and polypropylene as well as polyester, polyurethane, epoxy resin, silicone resin, polyamide, modified rosin, paraffin and waxes.
Typical examples of the coloring agent include carbon black, nigrosine, Aniline Blue, Chalcoyl Blue, Chrome Yellow, Ultramarine Blue, Dupont Oil Red, Quinoline Yellow, Methylene Blue Chloride, Phthalocyanine Blue, Malachite Green Oxalate, Lamp Black, Rose Bengale, C.I. Pigmentxc2x7Red 48:1, C.I. Pigment Redxc2x7122, C.I. Pigment Redxc2x757:1, C.I. Pigmentxc2x7Yellow 97, C.I. Pigmentxc2x7Yellow 12, C.I. Pigmentxc2x7Blue 15:1 and C.I. Pigmentxc2x7Blue 15:3.
Additives such as a known charge control agent and fixing adjuvant may be added to the toner as desired. The average particle size of the toner is 309 xcexcm or less, preferably 4 to 20 xcexcm.
The ratio of the toner, when the toner is mixed with the carrier to produce a developer, is preferably in the range of from 0.3 to 30% by weight of the total amount of the developer. Silica, alumina, tin oxide, strontium oxide, and a variety of resin powders, and conventionally known external additives may be added to improve the flowability of the developer.
A developing sleeve acting as the charging member can be obtained by coating the above resol-type phenol resin of the present invention on the outside surface of the cylindrical support to form the coating layer.
Conductive substrates, for examples, metals such as aluminum, stainless steel and insulating substrates, for example, ceramics and synthetic resins are given as the cylindrical support. Given as examples of the method for forming the above coating layer of the resol-type phenol resin on these cylindrical supports are a dipping method in which the substrate is dipped in a coating layer forming solution and a spraying method in which a coating layer forming solution is sprayed on the surface of a substrate. As the solvent used for the coating layer forming solution, any solvent may be used insofar as long as the above resol-type phenol resin dissolved in it. For example, toluene, tetrahydrofuran, dimethylformamide and chloroform may be used.
The film thickness of the coating layer of the developing sleeve acting as the charging member of the present invention is generally in a range of from 1 to 500 xcexcm, preferably from 5 to 300 xcexcm.
A resistance control agent such as carbon black, tin oxide and titanium oxide may be added to these coating layers to control the resistance.
In this case too, it is necessary to harden and crosslink the coating layer in the same manner as in the production of the aforementioned carrier. The heating temperature in this case is preferably 100xc2x0 C. to 200xc2x0 C. and more preferably 120xc2x0 C. to 180xc2x0 C. to harden the resol-type phenol resin sufficiently.
The developer layer regulating blade as acting the charging member of the present invention can be produced by applying the above resol-type phenol resin in the present invention to the surface of a blade substrate for regulating a developer layer. Given as examples of materials used as the substrate of the developer layer regulating blade are elastic materials such as rubbers, resins, and elastomers as well as materials similar to those of the developing sleeve.
The charging member of the present invention prepared in this manner may be used in an image forming method comprising a step of forming an electrostatic latent image on an electrostatic latent image carrying member, a step of forming a developer layer on a developing sleeve and a step of developing the electrostatic latent image on the electrostatic latent image support using the developer layer. In short, a developer using the carrier of the present invention as the charging member can be utilized as the above developer used in the aforementioned image forming method. Further, the above developing sleeve which is the charging member of the present invention can be utilized as the developing sleeve in an image forming appratus.
The image forming apparatus comprises a means of forming an electrostatic latent image on an electrostatic latent image support, a means of forming a developer layer on a developing sleeve and a means of developing the electrostatic latent image on the electrostatic latent image carrying member using the developer layer.